1. Field of the Invention
The present invention relates to cold and hot thermal energy storage systems as well as using such systems to optimize and reduce the electric and natural gas energy consumption of a building. In particular, this invention relates to a method of controlling the operation of a thermal energy storage device including a novel method of calculating the thickness of accumulated ice within the device.
2. Description of the Prior Art
Improving the energy efficiency of building comfort systems has become progressively more important due to rising energy costs as well as increased awareness and concern over global warming as a result of humanity's rising consumption of carbon fuels for electrical energy generation, direct burn heating, and domestic hot water appliances. One area where these concerns can be addressed is through leveling demand by shifting some of the load during peak hours of a day to off-peak times, thereby eliminating the need to build and run expensive peak generator turbines. These turbines are costly to build, install and operate and are typically used for only a very limited number of hours during the hottest days of the year. This invention addresses these concerns by increasing the efficiency of building comfort systems and helping to control electricity demand while increasing the general comfort level in the installed facilities.
Demand control and increased efficiency is primarily accomplished by shifting the burden of cooling from the hottest time of the day to the night when ambient temperatures as well as demand are considerably lower. Refrigeration equipment efficiency increases when the temperature lift requirement decreases. The difference in temperature lift between a hot day and a cool night can often be as high as 50%, thereby resulting in a massive drop in refrigeration equipment lift requirements and a corresponding efficiency increase. The problem is typically this equipment is required to operate during the day, due to the lack of cost-effective, efficient energy storage. And worse yet, the resulting demand in electricity consumption peaks requiring the use of low-efficiency gas turbine peak generators. The efficiency of these generators are generally 40-50% lower than good steam turbines which generate most of our electricity. Reducing or eliminating these peaks can be accomplished by storing energy for later use, and is the basic principle of Thermal Energy Storage (TES) technology. By storing cold water or ice during off-peak “cool hours” and then using this thermal energy to cool a facility during peak times will considerably reduced power consumption from the grid as well as helping to balance generating loads over a 24-hour period. In the same respect, hot water can also be generated during the daytime using, for example, solar and stored for later use in domestic hot water or nighttime heating.
While there are different types of thermal storage systems on the market the most common designs are based on cold water or two-phase ice/water storage. In recent years the ice storage systems have increased in popularity due to a considerably higher energy storage density. Currently ice storage systems are commonly used in large buildings and campuses. Such systems will generally contain chillers which cool a secondary heat transfer media such as brine to temperatures lower than water freezing temperatures. The brine circulates through tubes in ice storage tanks and cools water thereby generating ice. These systems are very complex, bulky, expensive and difficult to scale down for use in small commercial buildings or residential applications.
More recently a different approach to ice storage systems design was introduced. These systems generate ice through direct expansion of the refrigerant in a coil submerged in a tank of water. When it is time to use the accumulated cold energy the coil serves as a condenser in a secondary refrigerant loop where it condenses the refrigerant evaporated in an air conditioning unit coil. These types of systems are better suited for smaller buildings but they too suffer from being too complex, expensive and bulky for small businesses and homes. Moreover, such systems do not provide a high level of comfort due to their inherent deficiency of controlling temperature by cycling the system on and off. Also, these direct expansion systems can be used only for cooling purposes and are very difficult or impossible to combine with solar heating solutions which are becoming increasingly more important in energy conservation strategies.